1. Field
The present invention relates to a modified diene elastomer comprising a group bearing a function comprising a silicon atom predominantly hydrolysed into silanol and a tertiary amine function, and also to a process for synthesizing this diene elastomer. The present invention also relates to a reinforced rubber composition, which can be used especially in the manufacture of tires, comprising this modified diene elastomer.
2. Description of Related Art
Now that savings in fuel and the need to protect the environment have become a priority, it is desirable to produce blends that have good mechanical properties and hysteresis that is as low as possible in order to be able to use them in the form of rubber compositions that may be used in the manufacture of various semi-finished products included in the composition of tires.
Many solutions have already been explored to achieve the objective of lowering hysteresis. Mention may be made in particular of modification of the structure of diene polymers and copolymers by means of functionalization agents, with the aim of obtaining good interaction between the polymer thus modified and the filler, whether it is carbon black or another reinforcing filler.
Among numerous diene elastomer modifications, it has especially been proposed to improve the properties of rubber compositions by using elastomers functionalized with a silanol function.
Patent applications EP 0 778 311 A1, EP 0 786 493 A1 and EP 0 877 047 A1 have described a rubber composition reinforced using, respectively, as reinforcing filler, silica, carbon black and carbon black bearing silica fixed onto its surface, based on a diene polymer functionalized at the chain end with a silanol function or a polysiloxane block bearing at least one silanol end group or a diene polymer functionalized along its chain with silanol functions.
Patent application EP 2 139 703 proposes a functional elastomer having monomodal distribution of the number-average molecular masses of the chains, bearing a silanol function located essentially in the middle of the chain. This functional elastomer offers a composition containing it an improved compromise in terms of use in raw form/hysteresis especially relative to an elastomer essentially functionalized at the end of the chain with a silanol function.
It has also been proposed to combine the functionalization of diene elastomers by combining a silanol function with another function. Thus, patent application WO 2009/077 837 A1 describes an elastomer functionalized at one chain end with a silanol function, the other chain end bearing an amine function.
Moreover, patent application EP 2 266 819 A1 proposes, in view of its properties of attenuating heating and its synthesis limiting the production of volatile organic compounds, a diene elastomer functionalized at the end of the chain with a silanol function bearing a function promoting bonding between the silanol function of the elastomer and the reinforcing filler in a rubber composition.
Nevertheless, it appears that certain diene elastomers modified with a group bearing both an amine function and a silanol function pose problems of evolution of the macrostructures especially during storage, which leads to degradation of the potentially advantageous properties of these elastomers. Most particularly, it turns out that certain diene elastomers functionalized at the end of the chain with a silanol function bearing an amine function lead to large changes in Mooney viscosity.
The problem of evolution of macrostructure during the synthesis or storing of modified diene elastomers is known especially in relation with modified elastomers comprising alkoxysilane functions.
Numerous studies have focused on Mooney stabilization during the synthesis of this type of modified elastomer and during its storage. As an illustration, mention may be made of patent EP 0 801 078 B1 claiming the addition of a carboxylic acid to the elastomer solution, before the stripping step, in order to neutralize the basicity introduced by the anionic polymerization initiator and to minimize the hydrolysis of the alkoxysilane groups. Patent EP 1 198 506 B1 proposes to add a compound of alkyl alkoxysilane type (R1nSi(OR2)4-n) to the elastomer solution, before the stripping step, with a high (20/1) n(R1nSi(OR2)4-n)/n(Polymer-SiOR′) mole ratio so as to promote the reaction between the alkyl alkoxysilane and a part of the polymer that is in its totally or partially hydrolysed form, Polymer-SiOH, on stripping. Finally, mention may be made of patent EP 1 237 934 B1 which describes the addition of a long-chain alcohol to the elastomer solution, before the stripping step, so as to minimize the hydrolysis reactions on stripping and consequently the formation of Si—O—Si bonds during storage.
Nevertheless, these processes do not adopt the approach initiated in patent application EP 2 266 819 A1 of minimizing the impact of the synthetic process on the emission of volatile organic compounds.